In general terms, a circulator is a microwave device having the property of non-reciprocity. This means that the transmission of a circulator in one (forward) direction is much higher than that in the other (reverse) direction. In more specialized terms, the forward transmission loss is called insertion loss, whereas the reverse transmission loss is called isolation. Hence, a circulator has an intrinsic property that its insertion loss is much smaller than its isolation.
Most circulators have three ports (denoted port 1, port 2, and port 3 for brevity). In such a circulator, if the forward transmission traverses cyclically via port 1, port 2, and port 3 in sequence, the reverse transmission would traverse cyclically via port 3, port 2, and port 1 in sequence, and vice versa.
Circulators are widely used in radio frequency (RF) and microwave transceivers, such as in radio base stations. For example, an isolator (as represented by a circulator having only two ports) can be arranged between a transmitter and an antenna to protect the transmitter from antenna mismatch. In such a configuration, the forward power from the transmitter is passed to the antenna with low loss whereas the reflected power (due to possible antenna impedance mismatch) is heavily attenuated, so as not to affect the transmitter. Another typical example is to substitute the isolator by a three-port circulator for antenna transmission and reception duplexing. In this case, the transmitter signals are only passed to the antenna whereas any received signal from the antenna is only passed to the receiver. The reverse transmissions are greatly suppressed by the circulator.
Despite the large number of useful scenarios, the non-reciprocity essential to the circulator operation can currently only be found in two types of materials, namely ferrimagnets and plasma. The great majority of practical circulators are made from ferrimagnetic materials. However, alternatively, circulators can be made from active electronic devices, such as unilateral amplifiers made from biased transistors. In practice, due to the inferior noise, linearity, power consumption and power handling capabilities of transistors, biased transistors are rarely used to make circulators.
It has in “Magnetless Microwave Circulators Based on Spatiotemporally Modulated Rings of Coupled Resonators”, N. A. Estep, D. L. Sounas, and A. Alù, in IEEE Transactions on Microwave Theory and Techniques, vol. 64, no. 2, pp. 502-518, February 2016, been proposed to synthesize magnet-free circulators from variable capacitance diodes (also known as varactors). Variable capacitance diodes are a type of active electronic device. In such a design, a circular resonator is made from varactors resonating with inductors or transmission lines. The varactors are modulated by poly-phase sinusoidal signals with circular phase progression. As such, a forced time-varying device is made to permit wave propagation in both clockwise and counter-clockwise directions, but with different propagation velocity. As used hereinafter, the term forced time variance is defined as a behavior of a device such that its input-output relationship is forced to vary in time. The circulator made as such can be very compact and low-cost, and furthermore easily implementable on monolithic integrated circuits (IC), such as complementary-metal-oxide-semiconductor (CMOS) on silicon.
It has in “Magnetic-free non-reciprocity based on staggered commutation”, Negar Reiskarimian and Harish Krishnaswamy, in Nature Communications, published online Apr. 15, 2016, been proposed another magnet-free circulator synthesized from CMOS transistor switches. In such a design, the transistor switches are periodically switched, but in a sequence of staggering commutation, in order to achieve non-reciprocity. A transistor switch does not need a direct current (DC) bias, therefore it is often referred to as a “passive switch”. But a transistor switch is considered to be an active electronic device. Furthermore, the periodically driven switches have also forced time variance.
Conventional circulators, made from ferrimagnetic materials, are not compatible with semiconductor IC integration. Furthermore, it requires precision mechanical machining and assembly. As a result, ferrimagnetic circulators are bulky and expensive, thus unfavorable to high-volume, highly integrated applications such as antenna arrays.
Circulators made from DC biased transistors can be IC compatible. But DC biased transistors not only consume power but also generate both noise and non-linear harmonic and intermodulation produces. Furthermore, in high power applications, transistors limit power handling capability and efficiency.
The magnet-free circulators described above require a non-zero frequency modulation. It is therefore a forced time-varying device. One inevitable property of forced time-varying devices is the generation of intermodulation products, even if they are perfectly linear. More specifically, the magnet-free circulators described above unavoidably mix RF signals with its local modulation signals, effectively diverging RF power into other frequencies. This power diverging behavior limits the achievable insertion loss for this type of circulators.
Hence, there is still a need for an improved circulator device.